Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
Environmental changes and population growth are putting increasing pressure on water supplies as we enter the 21st century. Processes such as wastewater purification, sewage treatment and minerals processing rely upon the efficient removal of suspended solids in the form of fine colloidal particles with sizes in the sub-micron range. Brownian motion and small particle mass produce very slow sedimentation rates of these small particles when dispersed in a liquid (such as water), which make removal of those particles from the liquid a difficult operation. It is common practice that the particles are usually caused to aggregate to facilitate more rapid settling and easy removal of the solids by gravity. In an aqueous system, this can be achieved in a number of ways including the addition of excess electrolyte (coagulation), the addition of a high molecular weight polymer (bridging flocculation) and change of pH. Bridging flocculation is important in wastewater treatment and mineral tailings disposal.
In addition to rapid sedimentation, the properties of the resulting dewatered product (sediment or filter cake in the case of filtration) are important for the down stream processing. The consolidation behaviour and rheological (flow) of the sediment bed (including compressive yield stress PY and shear yield stress τY) are important in both the dewatering operation and the down stream processes. For instance, the final liquid content (moisture in the case of water) of the sediment should usually be minimised. Low moisture contents in sediments means that there is better solid/fluid separation efficiency, which is usually the desired goal. Quickly reaching that high separation efficiency is also desirable form an economic point of view. The compressive yield stress and the hindered settling factor are parameters that can be used to characterise the moisture content of the dewatered product and the rate of dewatering respectively. The viscosity and shear yield stress of the dewatered suspension are important factors in the pumping of the dewatered suspensions during processing and disposal.
For example the treatment of waste-water at a solids volume fraction of around 1% from a mineral processing plant are initially flocculated at the inlet to a thickener. The solids are then concentrated by gravitational sedimentation to the bottom of the thickener. The underflow from the thickener will have a solids volume fraction of +20%. This slurry is then passed through a high shear pump before further processing in a tailings pond to remove as much of the remaining moisture as possible. The clear water from the top portion of the tailings pond is typically recycled back to the processing plant for reuse. However, increased environmental pressures encourage the use of an in-line filtration step resulting in a ‘dry cake’ that is more easily transported and used as back fill. Ultimately more efficient solid/liquid separation results in both a lower volume of solids containing suspension to be disposed of and a greater fraction of recycled water.
Previous analysis of processes similar to the above can separate the process into a number of steps. These include settling, sediment consolidation, sediment pumping and filtration of the sediments. The precise demarcation between these steps is sometimes not clear and they are influenced to a large extent by the state of the initial suspension. The process of efficiently and rapidly separating the solid particles from a fluid and recovering the associated fluid phase is quite complex. It is possible to separately optimise the settling, sediment consolidation, sediment pumping and filtration steps by, for example, different chemical additives or adjustment of process variables such as pressure and shear rate. However, obtaining optimum properties for each step can typically be mutually exclusive when the solution conditions are maintained constant.
To explain, settling depends to a large extent on aggregate size and its density. Consolidation of the sediment bed occurs in response to an applied load. The efficiency of such consolidation depends upon the compressive yield strength of the bed. In sediment pumping, the flow of the sediment in response to a shear stress depends upon the shear yield stress and viscosity of the sediment particle network. Filtration of the sediment will depend upon the applied pressure, the network compressive yield strength and permeability/pore distribution in the bed.
As can be seen from the above, there are a number of competing interests when separating solids from solid/liquid suspensions.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.